It has previously been proposed to make pressure transducers in which capacitive plates are spaced apart and mounted on one or more ceramic diaphragms. Such transducers are disclosed, for example, in U.S. Pat. No. 4,064,550; and in U.S. patent application Ser. No. 898,518, filed Apr. 20, 1978, and assigned to the assignee of the present invention. As disclosed in the prior patent application, one plate of a capacitor may be mounted on a flexible circular ceramic diaphragm, and the other plate of the capacitor may be mounted in facing relationship on a heavier substrate. The diaphragm may be spaced from the substrate around its periphery by the use of glass frit, which not only spaces the diaphragm apart from the substrate but also seals the periphery of the diaphragm to the substrate. When pressure is applied to the outer surface of the diaphragm it deflects towards the substrate and the two facing conductive plates come closer together, thereby increasing the capacitance of the pressure sensing capacitor. In order to convert the capacitance changes to an electrical signal representing the difference in pressure, a fixed reference capacitor has been employed, and two frequency generation circuits were established with one using a fixed reference capacitance and the other using the pressure variable capacitor of the type mentioned above. Phase differences are caused by the change in capacitance of the variable capacitor, these differences are detected, and a d.c. voltage which is generally proportional to the capacitance change is generated.
In general, good linearity has been achieved using the circuit as described above, but at pressures above 100 kilopascals, the departure from linearity reached the level of about one percent to two percent. For convenience it is noted that 96 kilopascals is approximately equal to one atmosphere or one bar of pressure, which in turn is equal to about 14.7 pounds per square inch. While linearity to within 1 or 2 percent is sufficient for most purposes, it would be desirable to substantially reduce even this low level of non-linearity at higher pressures.
Accordingly, a principal object of the present invention is to improve the linearity of capacitive pressure transducers, and of systems utilizing such transducers.